The induction of specific transplantation tolerance, a state resulting in the acceptance of tissues from a particular donor type while maintaining otherwise normal immune reactivity, is a major goal of transplantation research. Transplantation of kidney allografts between various donor/recipient combinations have demonstrated the overwhelming importance of matching for the major histocompatibility complex (MHC) class II region in inducing specific tolerance to vascularized grafts in miniature swine, and similar conclusions may be drawn from clinical studies. During the previous project period we demonstrated that transfer of class Il genes, through retrovirus-mediated transduction of autologous bone marrow cells, could substitute for allogeneic bone marrow transplantation in inducing specific transplantation tolerance to subsequent fully mismatched renal allografts. Surprisingly, specific tolerance was successfully achieved by transferring the class Il DR locus alone. Although expression of the allogeneic DR transgene was low and limited to a fraction of bone marrow-derived cells, matching for DR alone appeared to control immunity to alloantigens displayed on the vascularized organ. In view of the powerful tolerogenic effects of DR in this model we hypothesize that induction of class II-dependent tolerance is a dominant phenomenon which may be mediated by identity for at least one class II product, and which only requires a low signal level (possibly peptides) present on an appropriate bone marrow-derived cell type in the early phase of the hematopoietic system reconstitution. In order to test this hypothesis we propose in this renewal to: l) Test whether identity to class II DQ alone can also control tolerance induction to primarily vascularized organ transplants in miniature swine; 2) Examine possible mechanisms leading to long-lasting unresponsiveness; 3) Characterize the bone marrow-derived cell type which induces tolerance via class II gene therapy; and 4) Examine the correlation between long-term expression of the transferred genes and long-lasting tolerance. Information obtained from these studies will be critical in defining the mode of action of the class II molecules in tolerance induction and will permit the design of appropriate protocols for future gene therapy approaches to tolerance of vascularized organs in man.